Impacts of Rainfall Intensity and Slope Gradient on Nitrogen Losses Processes Under Simulated Rainfall.

Nitrogen (N) loss due to soil erosion is one of the major concerns involved in non-point source pollution and land degradation. It is very important to figure out the intrinsic mechanisms of the N loss processes. Most former studies focused on the relationships among hydraulic parameters and sediment transport, while few researches studied the relationships among these parameters with nutrient loss processes. The objectives of this study were to investigate the impacts of rainfall intensity and slope gradient on erosion processes, N loss processes, flow hydraulic characteristics and dynamic mechanisms. Field rainfall simulation experiments were conducted with three rainfall intensities (25, 50, and 75 mm h^-1) and three slope gradients (5, 10 and 15°). Kastanozem was selected in the experiment. The results showed that soil loss rates and sediment-associated N loss rates increased with the increase of rainfall intensity and slope gradient. As for the runoff-associated N loss rates, it increased with the rainfall intensity. The velocity increased with an increase of either rainfall intensity or slope gradient, which resulted in the increase of Reynolds number, Froude number, shear stress, stream power and decrease of Darcy–Weisbach resistance coefficient and Manning roughness coefficient. Over all, rainfall intensity and slope gradient exhibited important impacts on soil erosion processes, N loss processes and hydrodynamic characteristics. Then the linear correlations were established to analysis the relationships among soil erosion, N loss processes and hydrodynamic parameters. It turned out to be that stream power was the best indicator for both sediment and sediment-associated N transport, while Reynolds number was the best indicator for runoff- associated N transport. This study helps understand the hydraulic mechanisms of the soil erosion processes and N loss processes.